on the surface vs sunken

on the surface

prep
  • apparently, outwardly; superficially 

sunken

adj
  • caused, by natural or unnatural means, to be depressed (lower than the surrounding area) or submerged 

  • Seeming to have fallen deeper back into the face due to tiredness, illness, or old age. 

How often have the words on the surface and sunken occurred in a corpus of books? (source: Google Ngram Viewer )